Dual layer etch stop barrier

ABSTRACT

A method for reactive ion etching of SiO 2  and an etch stop barrier for use in such an etching is provided. A silicon nitride (Si x N y ) barrier having a Si x  to N y  ratio (x:y) of less than about 0.8 and preferably the stoichiometric amount of 0.75 provides excellent resilience to positive mobile ion contamination, but poor etch selectivity. However, a silicon nitride barrier having a ratio of Si x  to N x  (x:y) of 1.0 or greater has excellent etch selectivity with respect to SiO 2  but a poor barrier to positive mobile ion contamination. A barrier of silicon nitride is formed on a doped silicon substrate which barrier has two sections. One section has a greater etch selectivity with respect to silicon dioxide than the second section and the second section has a greater resistance to transmission of positive mobile ions than the first section. One section adjacent the silicon substrate has a silicon to nitrogen ratio of less than about 0.8. The second section, formed on top of the first section is formed with the ratio of the silicon to nitrogen of greater than about 0.8. Preferably the two sections together are from about 50 to about 100 nanometers thick.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/031,251,filed Feb. 26, 1998, now U.S. Pat. No. 6,420,777.

BACKGROUND OF THE INVENTION

This invention relates generally to the manufacture of integratedcircuit (I/C) chips and particularly to the fabrication or processing ofa silicon substrate to form the circuitry for the I/C chip. During onestage of manufacture of I/C chips, a silicon dioxide layer is appliedover a silicon substrate. The silicon dioxide must be etched at variousplaces to provide openings to the substrate for electrical connections.One conventional technique of etching is by means of reactive ionetching (RIE). With reactive ion etching it is conventional to providean etch stop barrier between the silicon substrate and the silicondioxide layer formed thereon. One conventional etch stop barrier issilicon nitride (Si_(x)N_(y)). These silicon nitride barriers areconventionally deposited by low pressure chemical vapor deposition(LPCVD) utilizing conventional equipment. In one embodiment mixtures ofsilane (SiH₄) and ammonia (NH₃) are utilized as an ambient to providethe necessary silicon and nitrogen moieties for the formation of thesilicon nitride.

However, it has been found in the past that there were variations fromprocess to process of forming the Si_(x)N_(y) barrier in theeffectiveness of the nitride barrier in its selectivity with respect toSiO₂ when reactive ion etching the SiO₂. When etching SiO₂ it isdesirable to have as much selectivity as possible of the etch stop withrespect to the SiO₂ so as to allow a minimum thickness of the etch stopto be applied. It was also found that there were variations in theresulting barrier in the effectiveness of the silicon nitride to preventpassing of positive mobile ions (PMI) which may occur during subsequentprocessing due primarily to contaminants introduced into the SiO₂ layer.Positive mobile ion contamination (PMIC) such as in a gate oxide of CMOSdevices must be reduced to a minimum. Thus a requirement of the siliconnitride barrier is that it act to effectively block positive mobile ionsfrom penetrating into the substrate during subsequent processing steps.

Therefore it is desirable to provide a silicon nitride barrier that isboth highly selective to etching of SiO₂ and also effective to block thepassage of positive mobile ions in subsequent processing steps.

SUMMARY OF THE INVENTION

According to the present invention, a method for reactive ion etching ofSiO₂ with an etch stop barrier for use in such an etching is provided.It has been found that a silicon nitride (Si_(x)N_(y)) barrier having aSi_(x) to N_(y) ratio (x:y) of less than about 0.8 and preferably thestoichiometric amount of 0.75 provides excellent resilience to positivemobile ion contamination, but poor etch selectivity. However, a siliconnitride barrier having a ratio of Si_(x) to N_(y) (x:y) of 1.0 orgreater has excellent etch selectivity with respect to SiO₂ but a poorbarrier to positive mobile ion contamination. The technique of thepresent invention includes providing a substrate which conventionally isa doped silicon substrate, and forming a barrier of silicon nitride onthe substrate which barrier has two sections or layers. One section hasa greater etch selectivity with respect to silicon dioxide than thesecond section and the second section has a greater resistance totransmission of positive mobile ions than the first section. Preferablythe two sections are formed by forming one section, referred to as thelower section adjacent to silicon substrate with a silicon to nitrogenratio of less than about 0.8 and preferably about 0.75 which is thestoichiometric ratio of silicon to nitrogen. The second section, orupper section is preferably formed with the ratio of the silicon tonitrogen of greater than about 0.8 and preferably at least about 1.0.Preferably the two sections together are from about 50 to about 100nanometers thick and in the preferred embodiment, each section is about25 to 50 nanometers thick.

DESCRIPTION OF THE DRAWING

FIG. 1 is a graph of the etch rate of silicon nitride (Si_(x)N_(y)) inAr:CHF₃CF₄ at various silicon to nitrogen ratios (x:y) of the siliconnitride;

FIG. 2 is a bar graph showing the V_(t) shift of a substrate afterreactive ion etching using silicon nitride (Si_(x)N_(y)) barriers ofvarious ratios of silicon to nitrogen (x:y);

FIG. 3 is a graph similar to FIG. 2 graphing the positive ion density inthe substrate as a function of the ratio of the silicon to nitrogen(x:y) in silicon nitride; and

FIGS. 4A through 4G show the steps of the method of the presentinvention somewhat diagrammatically.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The use of silicon nitride as an etch stop barrier is well known in theart especially for stopping the etch during reactive ion etching (RIE)of silicon dioxide disposed over a silicon or doped silicon substrate inthe manufacture of integrated circuit chips. Reactive ion etching isused in chip manufacturing to form openings through the silicon dioxideso as to provide access to the substrate. Typically the opening will befilled with metal such as tungsten or other metal as is well known. Inetching the silicon dioxide an etch stop layer is used so as to allowthe etching to stop or essentially terminate once the etching haspenetrated through the silicon dioxide layer. Expressed another way,when the etching has pierced the silicon dioxide layer it is desiredthat the etching not continue to any significant extent. The barrierlayer of etch stop material is to ensure that the etch stopssubstantially uniformly at all the various locations being etchedthrough the silicon dioxide. Thus, one of the principal requirements ofthe etch stop material is that it have a relatively high selectivity ofetching with respect to the material which is intended to be etched i.e.silicon dioxide. Expressed another way, once the silicon dioxide hasbeen etched it is desirable that there be very little etching takingplace after that.

FIG. 1 shows the etching rate of Si_(x)N_(y) in nanometers per minuteusing an AME 5000 tool with Ar:CHF₃ atmosphere at various ratios ofsilicon to nitrogen in a silicon nitride (Si_(x)N_(y)) barrier. As canbe seen, when the ratio of silicon to nitrogen is 0.75 (which is thestoichiometric ratio) the etch rate is between 140 and 160 nanometersper minute, but as the ratio of silicon to nitrogen increases, this etchrate decreases dramatically to a point where when the ratio of Si to Nis about 1.0 the etch rate has dropped down to about 20 nanometers perminute. With a ratio greater than 1.0 no improvement in the etch rateresistance is achieved. Thus, based on this particular characterization,in order to get the lowest etch rate of silicon nitride and thus thehighest etch selectivity, it is desirable to have a ratio of silicon tonitrogen of at least about 1.0.

However, in subsequent processing during chip manufacture there can begenerated positive mobile ions (PMI), in particular Na⁺ and K⁺,principally from contamination in the SiO₂ layer. If these positive ionsdiffuse even in small amounts into the silicon substrate they can causesignificant degradation of the substrate material in some structures.Thus, it is desirable and often even necessary that these ions beessentially excluded from penetrating the barrier and diffusing into thesubstrate. FIGS. 2 and 3 show the amount of diffusion of positive mobileions especially sodium (Na⁺) as measured by Vt Shift (mV) shown in FIG.2 and ion density in 10¹⁰ Ions/cm² shown in FIG. 3 in substrates withSi_(x)N_(y) nitride barriers having various ratios of Si to N in thesilicon nitride. At a Si to N ratio of 1.05 there is a very high numberof mobile ions passing through the silicon nitride barrier, and even ata ratio of 1.0 there is an appreciable amount of these ions penetrating;indeed even at a ratio of silicon to nitrogen of 0.8 there is asignificant amount of PMIC (positive mobile ion contamination). It isnot until the ratio of silicon to nitrogen is 0.75 (i.e. thestoichiometric ratio) that the PMIC is essentially eliminated.

Thus, if one were to design the barrier to maximize resistance topositive mobile ion penetration one would use a ratio of silicon tonitrogen of 0.75. However, as shown above, this would provide very pooretch selectivity. On the other hand, if one were to design for the bestetch selectivity, one would design a nitride barrier having a ratio ofsilicon to nitrogen of 1.0 or greater; but this would provide poorresistance to positive mobile ion penetration.

According to the present invention, a barrier is provided which willachieve both high resistance to positive mobile ion penetration and verygood etch selectivity with respect to SiO₂. This is accomplished byproviding a barrier having two separate sections or layers. A firstlayer of silicon nitride is tailored to have excellent resistance topositive mobile ion penetration and thus has a ratio of silicon tonitrogen of less than about 0.8 and preferable about 0.75. A secondlayer of silicon nitride is provided which has a silicon to nitrogenratio of greater than about 0.8 preferably about 1.05. This will provideexcellent etch selectivity. By having a dual layer barrier as described,the barrier will provide not only good etch selectivity but resistanceto positive mobile ion contamination.

Referring now to FIGS. 4A through 4G, various steps of the presentinvention are depicted in very diagrammatic fashion. As seen in FIG. 4Aa silicon substrate 10 is provided which has a gate device 12 separatedfrom the substrate 10 by means of a gate oxide layer 13. The substratehas a region 14 of opposite polarity (shown as N⁺) on top of which is asilicided layer 15, which silicided layer 15 also overlies the gate 12.

A first layer of silicon nitride (Si_(x)N_(y)) 16 is deposited over thesubstrate 10 and the gate device 12. The first layer of silicon nitride16 in the preferred embodiment is formed in an AME 5000 tool sold byApplied Materials, Inc. with an atmosphere of SiH₄ and NH₃ to form asilicon nitride having a ratio of silicon to nitrogen of about 0.75. Theratio of silicon to nitrogen is controlled by controlling the ratio ofSiH₄ to NH₃ in a well known manner. Preferably this first layer 16 isfrom about 25 to about 50 nanometers thick.

Following the deposition of the first layer 16 a second layer 18 ofsilicon nitride is deposited over the first layer 16 as shown in FIG.4B. Again this is done in the AME 5000 tool in an atmosphere of SiH₄ andNH₃. The ratio of SiH₄ to NH₃ in forming this second layer 18 iscontrolled so as to form a silicon nitride with silicon to nitrogenratio of at least 1.0 and preferably 1.05. This layer 18 is also formedto a thickness of about 25 to about 50 nanometers so that the totalthickness of the first and second layers 16, 18 is from about 50 toabout 100 nanometers. It is not critical whether the layer 16 or 18 isformed on the substrate; however in the preferred embodiment, the layer16 is formed on the substrate 10 and the layer 18 is formed over thelayer 16.

On top of the layer 18 is deposited a layer of silicon dioxide (TEOS) 20preferably doped with boron (BSG) or phosphorous (PSG) or both (BPSG) asshown in FIG. 4C which also is formed in a conventional manner againusing the AME 5000 tool. This layer 20 is conventionally at least about0.6 microns thick.

As shown in FIG. 4D surface 22 of the TEOS 20 is coated with aphotoresist 24, which is photoimaged and developed in a conventionalmanner to provide openings one of which is shown at 26 in thephotoresist 22. One photoresist that is especially useful is positiveacting resist 5409 sold by Shipley Corp.

Following the developing of the photoresist layer 24, the SiO₂ exposedthrough the opening 26 is anisotropically etched preferably in a CHF₃:O₂atmosphere to form opening 28 in the SiO₂ as shown in FIG. 4E. Becauseof the layer 18 of Si_(x)N_(y) has a high Si to N ratio it has a veryhigh selectivity of etch rate as compared to the silicon dioxide 20, thelayer 18 Si_(x)N_(y) acts as an excellent etch stop material.Never-the-less a certain amount of the layer 18 is removed as shown as29 in FIG. 4E.

Following the reactive ion etching, the remaining photoresist 24 isstripped and the exposed silicon nitride layers 16 and 18 are removed bydry etching in Ar:CHF₃ to provide the structures shown in FIG. 4F.

Following the removal of the Si_(x)N_(y) layers in openings 26, acontact barrier such as TiN 30 is formed on the SiO₂ wall in opening 26and surface 22 and on the exposed substrate 10. This is followed bydeposition of a metal such as Tungsten (W) 32, as shown in FIG. 4G.

That portion of the Si_(x)N_(y) layers remaining under the SiO_(x),which have not been exposed and etched, contain the layer 16 which hasexcelled resistance to PMIC during subsequent processing. Thus the twolayers 16 and 18 have together provided high etch selectivity during RIEof the silicon and also reduced or eliminating PMIC during subsequentprocessing.

Of course it should be understood that the ratios of Si to N in the twolayers can be varied as can be the thicknesses of the two layers. Forexample if there is more concern for either more etch selectivity orimproved barrier to positive mobile ion penetration the thickness ofeach of the layers 16 and 18 as well as the ratios of Si to N in eachlayer can be varied. Also, as noted above the layer 18 with high etchselectivity can be formed on the substrate, and the layer 16 with goodresistance to PMIC can be formed on the layer 18.

Also it should be understood that in using a conventional tool forforming the silicon nitride, it is possible to provide a barrier whichhas a gradient throughout; i.e. a structure which at the surface of thesubstrate has excellent barrier properties to positive mobile ionpenetration and then gradually increases the silicon to nitrogen ratioso that the outer surface has high etch selectivity (or vice versa).This can be accomplished by starting with a ratio of SiH₄ to NH₃ thatwill provide a ratio of 0.75 of Si to N in the silicon nitride, and thengradually changing the concentrations of SiH₄ and NH₃ such that at theend of the cycle the ratio of Si to N in the silicon nitride is 1.0 ormore.

Thus, according to the present invention an improved etch stop barrieris provided which provides both excellent resistance to positive mobileion penetration and also very good etched selectivity in the samebarrier by having multiple layers of material which are tailored to aspecific function.

We claim:
 1. A method of reactive ion etching SiO₂ comprising the stepsof: providing a silicon substrate comprising a semiconductor deviceincluding a gate structure between a first and a second portions of saidsilicon substrate, forming a barrier of silicon nitride directly on saidsilicon substrate, said barrier of silicon nitride having a firstsection and a second section superimposed on each other and coextensiveentirely with each other with the first section being in contact withsaid first and second portions of said silicon substrate and the secondsection being spaced from said silicon substrate, said first and secondsections extending continuously from said first portion of said siliconsubstrate to said second portion of said silicon substrate over saidgate structure, said second section having a ratio of Si:N of at leastabout 0.8 and providing desired etch selectivity, said first sectionhaving a ratio of Si:N of less than about 0.8 and providing desiredresistance to positive mobile ion penetration, forming a layer of SiO₂on said barrier of silicon nitride, forming at least one opening throughreactive ion etching in said layer of SiO₂ using said barrier of siliconnitride as an etch stop layer, removing an exposed portion of saidbarrier of silicon nitride in said opening to reveal the substrate, anddepositing a conductor in said opening and in contact with saidsubstrate.
 2. The invention as defined in claim 1 wherein said firstsection has a ratio of Si:N of about 0.75.
 3. The invention as definedin claim 1 wherein said second section has a ratio of Si:N of at leastabout 1.0.
 4. The invention as defined in claim 3 wherein said firstsection has a ratio of Si:N of about 0.75.
 5. The invention as definedin claim 1 wherein said barrier is between 50 and 100 nanometers thick.6. The invention as defined in claim 5 wherein each of said first andsecond sections is between about 25 and about 50 nanometers thick. 7.The invention as defined in claim 1 wherein Si:N ratio in the barrier ofsilicon nitride progressively increases from said substrate through saidsecond section.
 8. The invention as defined in claim 1 wherein saidexposed portion of the barrier of silicon nitride is removed by dryetching.
 9. The invention as defined in claim 8 wherein the dry etch isperformed in an Ar:CHF₃ atmosphere.